A glass is a material that when cooled from a heated liquid transforms to the solid state without forming crystals. Such non-crystallized materials are also called amorphous materials. For example, one of the better known amorphous materials is quartz, which can be used to form conventional window glass.
Most metals crystallize when they are cooled from the liquid state at reasonable rates, which causes their atoms to be arranged into a highly regular spatial pattern or lattice. A metallic glass is one in which the individual metal atoms have settled into an essentially random arrangement. Metallic glasses are not transparent like quartz glasses and are often less brittle than window glass.
A number of simple metal alloys may also be processed to form a glass like structure. Binary metal alloys near deep eutectic features of the corresponding binary phase diagrams may be prepared into a glassy structure on cooling from the liquid state at rates greater than 1000 degrees per second. These binary metallic glasses may possess different properties than crystalline metals. These different properties may be useful in certain applications.
Bulk metallic glass forming alloys are a group of multicomponent metallic alloys that exhibit exceptionally high resistance to crystallization in the undercooled liquid state. Compared with the rapidly quenched binary metallic glasses studied prior to 1990, these alloys can be vitrified at far lower cooling rates, less than 10 degrees per second.
Many of the recently discovered bulk glass forming alloys can be broadly described as pseudo-ternary alloys of the form ETM1-x-yLTMxSMy. Typically the early transition metal couple, ETM, is a combination of elements from group IVB of the periodic table; e.g., Zr and Ti. The late transition metals, LTM, are typically combinations of the 3d transition metals from groups VIIIB and IB; e.g., Fe, Co, Ni, and Cu. The simple metal element, SM, is normally chosen groups from IIA or IIIA; e.g., Be, Mg or Al. However, the addition of a SM element is not a requirement for the formation of a bulk glass forming alloy.
Examples of some of the composition manifolds that contain ideal bulk metallic forming compositions are as follows: Zr—Ti—Cu—Ni—Be, Zr—Nb—Cu—Ni—Al, Ti—Zr—Cu—Ni, and Mg—Y—Cu—Ni—Li. There are also bulk metallic glass formimg alloys based on magnesium.
Each of the chemical species and their combinations are chosen for a given alloy composition, such that the alloy composition lies in a region with low-lying liquidus surface. Alloy compositions that exhibit a high glass forming ability are generally located in proximity to deep eutectic features in the multicomponent phase diagram.
The glass forming ability of a given alloy is in part described by the critical cooling rate that is required to avoid a fraction of crystal which is either large enough to be detectable, or large enough to cause some change of property. The glass forming ability is generally considered higher if the alloy composition has a reduced glass transition temperature. The reduced glass transition temperature is defined as the ratio between the glass transition temperature Tg to the liquidus temperature Tliq.
Early theoretical work on crystallization of undercooled liquid metals has showed that the nucleation rate was often vanishingly small for materials with Trg of approximately ⅔.
Bulk metallic glass alloys can be more easily formed if the eutectic like condition is satisfied. Many believe that the alloy should be close to a eutectic in order to obtain a high Trg.